In a digital computer system which includes an SCU with an associated main memory and multiple independent CPU's, each CPU with its own associated writeback cache, data stored in the SCU main memory may be invalid because more current data has been entered into one of the CPU's and stored into its associated CPU cache. A CPU in the system which requests selected data must find out if the version of the requested data stored in the SCU main memory is the most current version, or if not, the location of the CPU cache in which the most current version is stored. To this end, the SCU may contain a data storage location directory which includes a cache consistency unit (CCU) which indicates if the requested data stored in the SCU main memory has been invalidated, and the CPU cache location for the refreshed version of the requested data.
To provide the requesting CPU with data which is stored in the cache of another CPU, it has been necessary to read the data of the CPU cache in which the most current version of the requested data is located, transfer the read data back to the SCU, and then write the received data into the SCU main memory so that the received data replaces the less current version of the requested data already stored in the SCU main memory. After the refreshed data received from the located CPU is written into the SCU main memory, the SCU main memory is read so that the refreshed data can be transferred to the requesting CPU.
However, the above described procedure involves significant memory read latency because the SCU must first initiate a write cycle with a "WRITE" command to enter the refreshed data transferred to it into memory, and then, when the write cycle is complete, initiate a read cycle for the same data address with a "READ" command to transfer the most current version of requested data written in the SCU main memory to the requesting CPU. Thus, two complete SCU main memory cycles are necessary every time requested data is transferred to a requesting CPU from a cache of another CPU through the SCU.